At the 2021 AAEP Convention, Tom Divers, DVM, DACVIM, DACVSCC, of Cornell University, described notable changes in racehorse blood chemistry results. He emphasized that serum chemistry is more revealing than a CBC.
Elevated serum muscle enzyme activity is predictive of a muscle disorder, but it might not be predictive of poor racing performance. Lactate is also not a suitable predictor of race performance.
Some chemistries are telling:
- Clinical signs of muscle disease—stiff, decreased stride, reluctance to move, painful muscles post-race or training—strengthens the association of creatinine kinase (CK) and AST (aspartate aminotransferase) with poor performance.
- Marked elevations in CK and AST at 12-24 hours after racing—or prolonged increases in AST—might predict muscle disease being associated with poor performance.
- Significant increases in CK and AST post-exercise in fit horse warrants consideration of change in diet and training/racing protocols to reduce further muscle injury.
Divers noted that with an increase in AST and CK, it is important to look for muscle disease. If AST is increased but CK is normal and GGT is normal, then it is mostly likely muscle disease and can be checked with 15 minutes of a submaximal exercise test. However, if GGT is not normal, then check for liver disease, muscle and liver problems, and GGT syndrome (described below). Also, check other liver enzymes (i.e., SDH, GLDH, bilirubin and bile acids).
Divers described GGT syndrome as a condition of abnormally high serum GGT activity in racehorses without a notable increase in other liver or muscle enzymes.
In 2-13% of racehorses, there is a prevalence of a moderate increase in GGT of 50-140 u/L. GGT activity will decrease to normal range when training is reduced or stopped altogether.
As yet, there is no cause for this syndrome, but horses demonstrate reduced time to fatigue, a sour attitude, poor performance and weight loss.
GGT syndrome has been associated with cumulative training and racing in 3-year-old Thoroughbreds.
No differences were found in vitamin E concentrations, and selenium is within normal range in these horses.
In GGT syndrome horses, resting cortisol is elevated, indicating some level of stress. Bile acids also are somewhat elevated, as is total serum glutathione.
A possible explanation is based on enhanced glutathione recycling and mild cholestasis. High GGT syndrome might be a physiologic adaptation to maladaptation to the training load and oxidative stress. Racehorses with GGT >70 IU/l or levels 2-3 times the upper reference value likely need rest to recover. By decreasing a horse’s work output, GGT levels will return to normal.